1. Field of the Invention
With controllers, it is necessary to distinguish between intermittent and non-intermittent sensors or signalling units. In the case of intermittent signalling units or sensors, a functional test can be achieved with the aid of a plausibility test of the signal coming from these sensors or signalling units. This is not so, however, with the so-called non-intermittent sensors or signalling units, which supply an unchanged continuous signal over a long period of time. A simple example of such a sensor or signalling unit is a door contact switch, which in the cases both when the door is open and when it is closed emits a continuous signal as long as the operating state of the door does not change.
On the other hand, however, it is very important even in the very case of such signalling devices to determine whether they are still operating satisfactorily or whether damage has occurred and, possibly, an operating state is being signalled which no longer corresponds to the true conditions. An example in the case of a door contact switch would be the short circuiting between the two door contact switches in the case of a two-channel monitoring circuit. The consequence would be that only one door contact switch has to be actuated, while the position of the other door contact switch is no longer important. From the point of view of the evaluation circuit, both switches would be actuated as previously.
A further difficulty in the functional testing of the signalling units is caused by the amplitude or frequency filters normally required, the signal units being necessary satisfactorily to debounce and remove the interference from the signal coming from the signalling unit, in order to exclude damage to and maloperation of the inputs of the downstream evaluation circuit of the controller. These filters can have a substantial settling time. The signal at the output of the line which connects the signalling unit to the relevant evaluation circuit would have to be of a length corresponding to the filter settling time. This period is in conflict with the requirement to keep the testing time for the signalling unit as short as at all possible so that no dangerous operating states occur. To be precise, each extension of the testing time means that as long as the test is running the monitoring circuit does not detect if the signalling units signal a dangerous operating state of the monitored system. In addition, with multichannel monitoring devices the testing time is lengthened in accordance with the number of the channels, since all the signalling units monitoring the same station in a system must be tested simultaneously.
2. Description of the Related Art
By contrast, it is easier to handle transmission lines which are interconnected galvanically, transmit AC signals and, in addition, are connected at the end via transformers to the line drivers and line receivers. As shown in WO 94/00771, these lines can be tested with the aid of DC signals while the AC voltage transmission is proceeding simultaneously. Provided for this purpose at both ends of the line in each case is a balun which is connected at one end to the transmission line and at the other end to a symmetry transformer. Connected for the purpose of electrical interruption in the connecting line between the symmetry transformer and the balun is a capacitor which passes the useful AC voltage signal and to which the DC voltage used for testing is applied for testing purposes. This DC voltage can be tested for polarity and amplitude at the capacitor at the other end of the line. It is determined in this way whether the line is connected satisfactorily, interrupted or else has a cross.
Another possibility for testing a multi-core cable is described in the article "Timer IC's and LEDs for Cable tester", which appeared in "Electronics" 1973, pages 115 ff. A ring counter is connected to one end of a cable, while the other cable end is connected to one base of a transistor complementary stage. The other base is connected directly to the relevant output of the ring counter, which has a stage for each core in the cable. Depending on the routing conditions in the cable, there is a different current direction through a double LED connected in parallel with the transistors.
However, it is not possible using this test device to test the cable in the state of use. Rather, it has to be taken out of the transmission link.